Admittance spectroscopy for organic materials with Gaussian density of states

This work presents a systematic investigation of admittance spectra for single-carrier devices based on organic semiconductors featuring a Gaussian density of states. The dc current density is first numerically calculated using the master-equation-based mobility model. The small-signal admittance is subsequently computed from the first-order small-signal transport equations under the corresponding dc bias. The peak frequency fmax of the negative differential susceptance is systematically compared with the dc transit time τ. It is demonstrated that within the typical bias range, the product fmaxτ exhibits a weak dependence on both the applied bias voltage and the energetic disorder width, and remains nearly constant at approximately 0.69, close to the classical Mott–Gurney result for a device with constant carrier mobility. The combined analysis of admittance spectra and steady-state log–log current–voltage characteristics provides an effective approach to narrow the possible range of mobility parameters in disordered organic semiconductors.